At what RPM does the carbon start to burn off on a marine motor ?

What size motor are you talking about.

Johnson 225 High Output 1999

Carbon build up on spark plugs is a result fuel to air mixture not rpm's. When a 2 cycle is idling the fuel to air mixture tends to be rich do to the engine temperature is colder.

Tom

 

Air fuel ratio, engine temperature, oil fuel ratio, and quality of oil added all play a part in carbon buildup.  Using the correct plug in the right heat range can also be a factor.

On a properly tuned and jetted carburetor engine, the only time carbon should a problem is at long periods of idle.  A two stroke has to idle a little rich or it will lean spit, which is not good for reeds and other things.  Once you get up to above about 3,000 rpm you should be off the low speed jets and only running on mids.  At that point, after several minutes of running, the plugs should clean up a a paper bag brown or just a little darker toward the color of tea or coke.  They should not be burning black.  At approx 4,800 rpm than engine should start changing over to the high speed jets, depending on the size pullovers they have in the carbs and the plugs should start to lighten up a little more.  At WOT, 5,500-6,000 rpm,  they should be closer to a light tan to almost white.  If white, they are on the hairy edge to going lean, and that you don't want, can get expensive.

Now, with all that said, and since I know a little bit about reading plugs and tuning two strokes, I run mine a lot leaner than anything you are going to see come from the factory, makes a huge difference in fuel economy, but if the average individual try's that and the first cool, brisk morning he goes ripping off down the lake and is thinking to himself, man that thing is running good, it probably won't be for very long.  They make a distinct and very sickening sound when they pop a piston.

Also understand, a rich engine has more torque, a lean engine has more speed.

One other tid bit.  If you do a lot of wot running, run 78s in it, if you rarely make long, wot runs, use 77's in it.  I think those are QL77 or 78, old fart brain, just made a memory dump.

4 minutes ago, Way2slow said:

On a properly tuned and jetted carburetor engine, the only time carbon should a problem is at long periods of idle.  A two stroke has to idle a little rich or it will lean spit, which is not good for reeds and other things.  Once you get up to above about 3,000 rpm you should be off the low speed jets and only running on mids.  At that point, after several minutes of running, the plugs should clean up a a paper bag brown or just a little darker toward the color of tea or coke.  They should not be burning black.  At approx 4,800 rpm than engine should start changing over to the high speed jets, depending on the size pullovers they have in the carbs and the plugs should start to lighten up a little more.  At WOT, 5,500-6,000 rpm,  they should be closer to a light tan to almost white.  If white, they are on the hairy edge to going lean, and that you don't want, can get expensive.

Now, with all that said, and since I know a little bit about reading plugs and tuning two strokes, I run mine a lot leaner than anything you are going to see come from the factory, makes a huge difference in fuel economy, but if the average individual try's that and the first cool, brisk morning he goes ripping off down the lake and is thinking to himself, man that thing is running good, it probably won't be for very long.  They make a distinct and very sickening sound when they pop a piston.

Also understand, a rich engine has more torque, a lean engine has more speed.

That pretty well covers anything you ever wanted to know about spark plugs. 

Understand, what I posted only applies to two stroke carburated engines and those plug numbers were for a 3.0 V6 OMC. 

Four stroke engines and EFI/DFI two strokes are computer controlled so they are going to run much leaner and the plugs are going to be a lot closer to white, if not white.